I know what causes this, and have seen it myself with Droid Sans Mono. Due to the current design of the code editor, it's not an easy fix.

The problem comes from the way ClearType bleeds fonts over outside of their normal boundaries. When the editor has a color change, it draws a background rectangle first, which erases the part of the letter that bleeds over. This happens on wide characters like M's and W's, which push to the edge of their allotted space. This is why you see a blue line; it's ClearType doing that.

To fix this properly I'd need to figure out a different way of drawing the background rect, which also impacts selections.